This invention relates to operational amplifiers operated in class AB mode and more particularly to a combination capacitor structure and transistor circuit for providing bias to the output stages of the operational amplifiers for facilitating class AB modes of operation.
Contemporary high performance operational amplifiers generally are operated in a class AB mode and comprise a cascaded output section including an NPN and PNP output transistors. The PNP output transistor is usually coupled to a bias circuit which biases the transistor in a class AB mode of operation. Thus, the amplifier is characterized by low current drain and good fidelity. Examples of high performance operational amplifiers which may be operated in class AB modes are the MC1458 and MC1741 operational amplifiers manufactured by Motorola, Inc. These operational amplifiers use a compensation capacitor and pole splitting techniques to set the transfer characteristics of the amplifiers. The compensation capacitor is typically anywhere from 5-30 picofarads and is constructed integrally with the monolithic integrated operational amplifier. In these operational amplifiers substantial semiconductor die real estate is required for the capacitor which may be a disproportionate portion of the total semiconductor area required for the circuit. Furthermore, contemporary operational amplifiers usually utilize an NPN device for biasing the PNP output device in a class AB mode. This creates an inherent mismatch between the two devices which may require more current drain through the output stage than desirable and with less predictable currents being defined which in turn affects the operation of the operational amplifier.
In the design of high performance operational amplifiers it is very desirable to reduce the current drain through the output stages as much as possible to reduce the amount of total drain current required by the amplifier. However, even though the goal is to reduce the current drain in the output stage, it is also important that predictable currents be obtained. Moreover, as the tendancy is to make integrated circuits more complex, it is significant to be able to reduce the die surface area which is required to fabricate an operating circuit.
In view of the above, a need exists for providing a bias circuit for high performance operational amplifiers which can reduce the current drain through the output stage as well as reducing the die surface area required for fabricating the operational amplifier circuit.